Propeller having angularly disposed tip

ABSTRACT

A propeller which is given increased efficiency and rendered more silent in operation by shaping its tip edges to have, in addition to the usual pitch angularity, a second angularity causing the tip edge to advance progressively radially outward as it advances circularly, and specifically at a critical optimum angle of 71.5 degrees with respect to a line drawn directly radially outwardly from the axis of the propeller and intersecting the tip edge.

United States Patent 1191 Van De Water Dec. 17, 1974 [5 PROPELLER HAVINGANGULARLY 3,399,731 9/.1968 0116s 416/228 DISPOSED TIP 3,467,l97 9/1969Spivey et al. 416/228 [76] Inventor: Frank Van De Water, 28604 GN ATE TOR APPLICATIONS Covecrest Dr, Palos Verdes, Calif- 119,463 5/1919 IGreat Britain 416/223 90274 527,323 7/1921 France 416/223 [22] Filed:May 7,1973

Primary Examiner-Everette A. Powell, Jr. pp N093 357,835 Attorney,Agent, or FirmWilliam P. Green Related US. Application Data I I i [63]Continuation-impart of Ser. No. l59,l84, July 2, ABSTRACT 1971 abandonedA propeller which is given increasedefficiency and rendered more silentinoperation by shaping its tip 4lfi/22ggilfill2lig edges to have inaddition to the usual pitch angular [58] .ld .4l6/2;3 228 y Second g arty caus ng the p g to ad- 0 care vance progressively radially outward asit advances circularly, and specifically at a critical optimum angle[56] References cued of 71.5 degreeswith respect to a line drawndirectly UNITED STATES'PATENTS radially outwardly from the axis ofthepropeller and 2,345,047 3/1944 Houghton 1. M67223 intersecting thetip edge.

2,660,401 ll/l953 Hull .1 416/243 3,167,129 1/1965 Shultz 416/226 3Clams, 8 DraWmg'Flgul'eS Q 51F p 9- A,

25 W t b 1 1, \E g I 20 l 7 I PROPELLER HAVING ANGULARLY DISPOSED TIPCROSS-REFERENCE TO RELATED APPLICATION This application is acontinuation-in-part of my copending application Ser. No. 159,184 filedJuly 2, 1971 on Propeller Having Angularly Disposed Tip now abandoned.

BACKGROUND OF THE INVENTION This invention relates generally to improvedairplane propeller structures, especially designed to attain increasedefficiencyand more silent operation as compared with conventionalpropellers currently in use.

As a conventional aircraft propeller moves through the air, a verysubstantial drag occurs at the tip of the propeller blade, resulting inan ineffective waste of power, and development of a noticeable tip noiseproduced by the turbulence accompanying the waste of energy. I havefound that this inefficiency and unwanted tip noise are caused by aspilling over of some of the air from the back side of the propeller,past the tip of the blade, and to the front side of the propeller, in amanner destroying the forward lift on a substantial portion of the tiparea thus affected. When the tip is of the conventional square cut type,that is a type in which the edge of the tip extends directlyperpendicular to a radial line extending outwardly from the axis of thepropeller, the wasted portion of the tip constitutes a triangular regionof substantial area. Since the horsepower derivable from differentportions of a propeller blade increases rapidly as the blade advancesoutwardly toward its tip and in fact increases in proportion to a curveof the fourth power, any loss of effective area at the tip of the bladeis of extreme importance from an efficiency standpoint.

SUMMARY OF THE INVENTION A propeller constructed in accordance with thepresent invention is designed to substantially completely eliminate theabove discussed spillage or flow of air from the high pressure back sideof a propeller to its lower pressure forward side, to thereby avoid thementioned loss of power and efficiency, and even more importantly formany installations, to eliminate tip noise and render the blade silentin operation. This result is achieved by special formation of the tip ofthe blade so that its outer edge gradually advances radially outwardlyas it advances circularly'from the leadingedge to the trailing edge ofthe blade. The angle of outward advancement. of the tip edge correspondsexactly to the outward angularity at which I have discovered pressurizedair normally flows along the back side of the propeller when thepropeller is in operation under a heavy load, and more specifically isat a critical angle of 71.5 degrees with respect to a line drawndirectly radially outwardly from the axis of the propeller andintersecting the tip edge. As a result of this angularity, the air atthe back side of the propeller is effectively prevented from flowingforwardly past the tip to the front side of the blade.

The discussed outer tip edge of the blade, besides having this outward'angularity, also has the usual pitch angularity, with respect to aplane disposed transversely of the rotary axis of the propeller, and thepropeller has a twist as it advances radially outwardly. The twoangularities and the twist together result in an overall airplanepropeller structure which is far superior, from both an efficiency andnoise standpoint, to all prior propellers of which I am aware.

BRIEF DESCRIPTION OF THE DRAWING The above and other features andobjects of the invention will be better understood from the followingdetailed description of the typical embodiments illustrated in theaccompanying drawing, in which: FIG. 1 is a front view of an aircraftpropeller constructed in accordance with the invention;

FIG. 2 is a side view of the propeller, taken on line 2-2 of FIG. 1; v I

FIG. 3 is a greatly enlarged view of the propeller taken of line 33 ofFIG. .2, and showing it as it appears looking directly radially inwardlytoward the axis of the propeller from a location beyond its tip;

FIG. 4 is a view of the tip taken on line 4-4 of FIG.

tional blade tip cross section.

DESCRIPTION OF THE PREFERRED I EMBODIMENT Referring firstto FIG. 1, Ihave shown at, 10 a propeller embodying the invention which is to beutilized for propelling an airplane-forwardly in flight, and which has acentral hub portion 11 adapted to be mounted on and be driven by a shaft12 which turns about a generally horizontal axis 13. In FIG. 2, theaircraft engine which turns the propeller is representeddiagrammatically at 14. The propeller is typically illustrated as one offixed pitch, though it will be understood that the novelty of theinvention may of course be applied to a variable pitch propeller.Further, the propeller may have more than the typically illustrated twoblade 15.

. The propeller is driven in a counterclockwise direction as viewed inFIG. 1. Each of the blades 15 (both or all vof which are identical) isdisposed at a pitch angle (see FIG. 3) with respect to aplane 16disposed directly transversely of the axis 13 of the propeller. Theblade has a slight twist as it advances radially outwardly, so that thepitch angle decreases from the angle a of FIG. 3 at the innermostportion of the blade to the angle a at its tip 17. Throughout thisentireradial extent, the blade may have a conventional airfoil crosssection,,defined by a flat surface 18 at the back side of the blade anda slightly convexly curved forward surface 19 of the blade. These twosurfaces 18 and 19 ing generally circularly from the location of leadingedge 20 to trailing edge 21, the tip edge 22 alsoadvances progressivelyradially outwardly, as seen clearly in FIGS. 1 and 4, so that in effectthe blade becomes longer as it approaches trailing edge 21. Edge 22 maylie in the plane of the radially outermost portion of planar rearsurface 18 of the blade, with the forward face 19 of the blade beingcurved or rounded as seen at 25 in FIGS. 5 and 5a to merge with planarsurface 18 at edge 22. The discussed radially outward advancement of tipedge 22 in a direction away from axis 13 of the propeller continuesthroughout substantially the entire length of edge 22.

To describe the shape of outer tip edge 22 even more specifically, it ispreferred that at each individual point along edge 22 that edge bedisposed at a critical angle of 71.5 with respect to a line extendingdirectly radially outwardly from axis 13 and intersecting thatparticular point on edge 22. For example, with respect to theapproximately central portion of edge 22 which has been designatedby thenumeral 26 in FIG. 4, the men-. .tioned radial line extending directlyradially outwardly to that point from axis 11 is shown at 27, and thediscussed critical angle between tip edge 22 and line 27 is designatedby the letter b. Similarly, for the previously mentioned point 24 at thejuncture between edge 22 and trailing edge 21, the directly radial linepassing through that point from axis 13 is represented at 27 4 edge 22,as indicated at 30, and flows forwardly to the front low pressure sideof the blade in a manner de- Y stroying or reducing the front to rearpressure differential and therefore the forward thrust produced at that.

spillover location, and providing the elements of a vortex. The overallresult is a loss of forward thrust over a triangular area of the tipportion of the blade as represented at 38 in FIG. 6. This reduces theefficiencyof the blade and causes an undesirable noise at .the tip.

In the improved tip of FIG. 4, edge 22 is disposed at exactly the sameangle (b, b, etc.) as is followed by the air along paths 32 at the backsideof the propeller (corresponding to paths 29 of FIG. 6), so that edge22 actually parallels these paths of air movement, and the air at theback of the blade can never reach and spill over edge 22 before arrivingat the trailing-edge 21 of the blade. This prevents both theinefficiency and noise which occur in the FIG. 6 arrangement.

As indicated previously, for optimum results the angles b, b, etc. inFIG. 4 should be 71 .5, which angularity I have found correspondsprecisely to the angularity of air movement relative to the tip portionof the blade when working-at maximum effort. To the extent that theseangles b, b", etc. may be more than the optimum 71.5, some of the airwill be permitted to spill forwardly over the outer edge 22 of theblade, and thereand the critical angle at that location is designated b.

Both of angles b and b, as well as all other corresponding angles atlocations across substantially the entire length of edge 22, are thespecified 71.5".

It will be apparent that if the ideal angularity of the tip edge ismaintained continuouslyalong the entire length of edge 22, that edgevvill theoretically follow 'a ve ry slightly curvedarc between points23-alnd 2 1 This theoretical are, however, has so little curvature thatit maybe considered as a substantially straight line, and may in actualpractice be formed as a completely straight line, without causing thediscussed critical angularity to vary appreciably from the desired 7l.5.Any appreciable deivation from this 71.5 angularity results in adecrease in efficiency and an increase in noise at the propeller tip.

In order to bring out the reason for the angularly disposed outer tipedge 22 in FIGS. lto 5, I have illustrated at 28 in FIG. 6 the outer tipportion of a conventional square cut aircraft propeller, which may beconsidered as-identical with the propeller of FIGS. 1 to 5 except thatthe outer tip edge 22' (corresponding to edge 22 of FIG. 4) is notdisposed angularly, but rather is precisely perpendicular to a line-27'extending directly radially outwardly from the axis of the propeller.When a propeller of this FIG. 6 type is driven about its axis, say withthe tip moving leftwardly in FIG. 6, it is found that as the tip movesthrough the air, the air which engages the back side of the tip (thesurface corresponding to surface 18 of FIG. 3) progressively movesangularly radially outwardly as it passes over that rear surface. Theangular path which this air follows is represented by the lines 29inFIG. 6, which are disposed at an angle c with respect to the radial line27. Because of .this angularly outward movement of the air relative tothe engaged back side of the blade, some of the air which passes theleadingedge of the blade reaches the outer tip edge 22' before itreaches the trailing edge 21', and as'a result spills over the tip foresome inefficiency and noise are developedLOn the other side of the coin,to the extent that the anglesb, b, etc. are permitted to be less thanthe optimum 715, a portion of the tip which'would otherwise be effectivein producing forward thrust has needlessly been removed, and in this wayreduces efficiency for a propeller of given diameter.

It should also be noted that, while the pressurized air at the back sideof theblade follows the angular outwardly advancing paths represented at32 in FIG. 4, the lower pressure air at the front of the blade does notadvance radially outwardly in this manner, but rather moves directlyacross the blade along paths such as those represented at 33 in FIG. 4,which paths are substantially perpendicular to the radial center line 27of the blade (or more precisely are very-slightly butimperceptiblycurved and at each point normal to a radial line extending through thatpoint); The angular tip edge 22 of the blade functions as a leading edgewith respect to this air at the front of the blade, and for maximum effciency the blade is given an airfoil section in each of the planes inwhich the air at the front of the blade moves. FIG. 5 is a section takenin one-of these planes (normal to radial center line 27) and shows theairfoil section in that plane, consisting of a portion of that planarsurface 18 at the back of the blade and a portion of the convexly curvedsurface 19 at the front of the blade. The leadingportion 19of thisairfoil section is slightly rounded, as shown, .while the trailingportion adjacent edge 21 may be essentially sharp.

Without specifically illustrating theairfoil section in other planes, itwill be understood that in all planes which are parallel to the plane ofFIG. 5 and which in tersect outer edge 22 the tip of the blade has asimilar airfoil section of the general type illustrated in FIG.- 5, tothus maximize the efficiency of the tip portion of the blade.

' In the Figures, the outer tip'edge 22 is illustrated as meeting theleading and trailing edges 20 and 21 in abruptor sharp corners at 23 and24 as viewed in FIG.

4. In actual practice, it will of course be understood that thesecorners may be slightly rounded to facilitate handling of the bladewithout injury.

In FIG. 3, the direction of flight of the blade tip through the air(including the forward component of blade movement) is represented bythe arrow 35. The lines 32 and 33 in this Figure represent certain ofthe paths of air flow 32 and 33 of FIG. 4, at the back and front sidesrespectively of the blade. The angle of attack of any particular portionof the blade in FIG. 3 with respectto the contacted air is designated asthe angle D in FIG. 3, that is, the angle between arrow 35 and the plane36 of rear face 18 of that particular portionof the blade. In a fluid,the maximum value which this angle of attack, angle D, can have beforestalling is 18.5". The angle of stall in fluids has a slope of l to 3,and therefore a tangent of 0.333, and hence is 185. When the blade isoperating at maximum effort, the blade will be disposed at this 18.5angle of stall. I

Relating this angle D of FIG. 3 to the showing of FIG. 4, angle E inFIG. 4 may be termed the angle of displacement, and represents the anglethrough which the air is deflected (in the FIG. 4 plane) when contactedby the rear face 18 of the blade. This angle of displacement-E in FIG. 4is'always equal to the angle of attack D of FIG. 3, up to the maximumeffort angle of attack of 18.5", beyond which point stall conditions setin as indicated. Thus, the angle of displacement E in FIG. 4 is 18.5degrees at maximum effort,'and the optimum value for angle F in FIG. 4is 18.5 degrees, the comple-1 ment of the previously discussed critical71.5 angle b by which the angularity .of edge 22 has been defined atmost points in this specification.

As discussed previously, one reason for the loss of efficiency in theconventional FIG. 6 type of blade results from the loss of pressuredifferential between the front and back of the blade across thetriangular area which i has been designated 38 in FIG. 6. Another reasonfor loss of efficiency, however, results from the development of avortex circularly behind trailing edge 21 of the blade, because theangle A between the air leaving the front and rear sides of the blade isin excess of 18.5 degrees. In FIG. 4, on the other hand, this angularitybetween the two streams of air is the angle E, which'is l8.5 at maximumeffort and will not develop a vortex.

FIG. 7 represents a variational form of the invention, and in particularshows the sectional configuration of this variational form in the planein which FIG. 5 is taken. Except with respect to this blade crosssection, the propeller of FIG. 7 may be considered as identical withthat of FIGS. 1 to 5. For example, the tip edge 122 of the FIG. 7 bladeis of course disposed at the same 71.5 degree critical angle as is edge22 of FIG. 4, with respect to a radial line such as that shown at 27 inFIG. 4 (angle b in FIG. 4) and the blade has the same pitch angle,twist, and other structural features as in FIGS. 1 to 5, except for theillustrated difference in blade cross section. For most applications,the FIG. 7 contour has been found preferable to that of FIG. 5 formaximizing blade effeciency and minimizing air turbulence.

In FIG. 7, the airfoil section between the angular tip edge 122(corresponding to edge 22 of FIG. 4) and the trailing edge 121 of theblade (corresponding to edge 21 of FIG. 4) is shaped as a fifty percentairfoil. Stated differently the plane 34 in which the airfoil sectionhas its maximum thickness t between rear surface 118 and surface 119 islocated half way between tip edge 122 and trailing edge 121. Further,the leading portion of this airfoil section,1at edge 122, is much'thinner than in FIG. 5, and in fact tapers to a substantially sharp edgeat 122. Similarly, trailing edge 121 is substantially sharp. In thususing the term sharp in describing edges 121 and 122, I intend toindicate that these edges are as thin as is practical in a blade of thistype, though it will of course be understood that a knife sharpcondition should preferably be avoided in order to prevent injury topersons handling the propeller. Thus, the edges 121 and 122 may berounded or curved at an extremely small radius, say for instance about0.020 of an inch, with the edges then typically having an overallthickness of about 0.040 of an inch. The thickness of the FIG. 7 crosssection increases'only gradually as it approaches the central plane 34from edge 122, and then preferably decreases at the same gradual rate between plane 34 and trailing edge 121, to thus be symmetrical withrespectto plane 34. The rear face 118 as viewed in FIG. 7 follows astraight line in extending between edges 122 and 121, in correspondencewith the similar rear face 18 of FIG. 5. Rear face 118 is thereforesubstantially planar, except with respect to the I very slight curvatureresulting from the gradual radial twist of the blade. r

The variational form of blade whose cross section is represented in FIG.7 has a sharp edged 50 percent airfoil section of the type illustratedin FIG. 7 in all planes parallel to the plane of FIG. 5 andperpendicular to radial line 27 of FIG. 4 (e. g. in planes containingthe vari ous lines 33 of FIG. 4). This is true of all such crosssections at any point throughout the entire radial extent of the blade,including sections near the tip of the blade which extend between anyportion of tip edge 122 and the corresponding portion of trailing edge121, and also similar sections extending'between edges 122 and 121 v atlocations radially inwardly of the tip portion. All such cross sectionshave the same appearance and proportions as are represented in FIG. 7,though the actual length of the different cross sections will of coursevary in accordance with the actual distance between the edges of theblade in the planes of the different sections.

It is found that this 50 percent airfoil of FIG. 7, with a sharp leadingedge, maintains'as nearly as possible constant acceleration of the airat the forward side of the blade, and-coacts with the 7l.5 angularitypreviously discussed in optimizing the operational characteristics ofthe blade.

While certain specific embodiments of the present invention have beendisclosed'as typical, the invention is of course not limitedto theseparticular forms, but

rather is applicable broadly to all such variations as fall within thescope of the appended claims.

- I claim:

1. A propeller which is to be power driven rotatively about apredetermined generally horizontal axis and in v a predetermineddirection to cause horizontal advance ment of an airplane: saidpropeller having a blade projecting generally radially outwardly awayfrom said axis and-which has a leading edge, and a trailing edge, and atip edge extending therebetween; said tip edge being disposed at a pitchangle with respect to a plane extending perpendicular to said axis ofthe propeller; said leading and trailing edges having outer portions atsubstantially their outermost ends which extend approximately radiallywith respect to said axis and which essentially meet opposite ends ofsaid tip edge; said tip edge being so shaped that, as it advancesgenerally circularly from saidapproximately radial outer portion of theleading edge to said approximately radial outer portion of said trailingedge, the tip edge also advances progressively radially outwardly awayfrom said axis, throughout essentially the entire circular distancebetween said approximatelyradial outer portions of said leading andtrailing edges, and at an angle of 7 1 .5 with respect to a lineextending directly radially outwardly from said axis and intersectingthe tip edge; said blade in extending between said tip edge and saidtrailing edge having an airfoil cross section in all planesperpendicular to a' radial line and intersecting both the tip edge andtrailing edge, which airfoil cross section is defined by a convexlycurved forward face of the blade and a substantially fiat rear facefollowing a straight line between said tip edge and said trailing edge;said airfoil cross section, in all of said planes, tapering to thinsubstantially sharp shapes at both the tip edge and trailing edge of theblade.

2. A propeller which is to be power driven rotatively about apredetermined generally horizontal axis and in a predetermined directionto cause horizontal advancement of an airplane: said propeller having ablade projecting generally radially outwardly away from said axis andwhich has a leading edge, and a trailing edge, and a tip edge extendingtherebetween; said tip edge being disposed at a pitch angle with respectto a plane extending perpendicular to said axis of the propeller; saidleading and trailing edges having outer portions at substantially theiroutermost ends which extend approximately radially with respect to saidaxis and which essentially meet opposite ends of said tip edge; said tipedge being so shaped that, as itadvances generally circularly from saidapproximately radial outer portion of the leading edge to saidapproximately radial outer portion of said trailing edge, the tip edgealso advances progressively radially outwardly away from said axis,throughout essentially the entire circular distance be-' tween saidapproximately radial outer portions of said leading and trailing edges,and at an angle of 7 1 .5 with respect to a line extending directlyradially outwardly from said axis and intersecting the tip edge; saidblade in extending between .said tip edge and said trailing edge havingan airfoil cross section, in all planes perpendicular to a radial lineand intersecting both the tip edge and trailing edge, which airfoilcross section is defined by a convexly curved forward face of the bladeand a substantially flat rear face following a straight 8 line betweensaid tip edge and said trailing edge; said airfoil cross section, in allof said planes, being a fifty percent airfoil cross section having itsmaximum thickness at a point midway between said tip edge and saidtrailing edge, and tapering to thin substantially sharp shapes at boththe tip edge and trailing edge of the blade.

3. A propeller which is to be power driven rotatively about apredetermined generally horizontalaxis and in a predetermined directionto cause horizontal advancement of an airplane: said propeller having ablade projecting generally radially outwardly away from said axis andwhich has a leading edge, and a trailing edge, and a tip edge extendingtherebetween; said tip edge being disposed at a pitch angle with respectto a plane extending perpendicular to said axis of the propeller; saidleading and trailing edges having outer portions at substantially theiroutermost ends which extend approximately radially with respect to saidaxis and which essentially meet opposite ends of said tip edge; said tipedge being so shaped that, as it advances generally circularly from saidapproximately radial outer portion of the leading edge to saidapproximately radial outer portion of said trailing edge, the tip edgealso advances progressively radially outwardly away from said axis,

throughout essentially the entire circular distance between saidapproximately radial outer portions of said leading and trailing edges,and at an angle of 71 .5 with respect to a line extending directlyradially outwardly from said axis and intersecting the tip edge; saidblade in extending between said tipv edge and said trailing edge havingan airfoil cross section, in all planes perpendicular to a radial line'and intersecting both the tip edge and trailing edge, which airfoilcross section is defined by a convexly curved forward face of the bladeand a substantially flat rear face following a straight line betweensaid tip edge and said trailingedge; said airfoil cross section, in allof said planes, being a per- I cent airfoil cross section having itsmaximum thickness at a point midway between said tip edge and saidtrailing edge, and tapering to thin substantially sharp shapes at boththe tip edge and trailing edge of the blade; said blade also having anairfoil cross section through at least a portion of its radial extentradially inwardly of said tip edge, in planes intersecting both saidleading edge and saidtrailing edge, which airfoil cross seciton isdefined by a convexly curved forward face and an essentially flat rearface following a straight line between said leading edge and saidtrailing edge.

1. A propeller which is to be power driven rotatively about apredetermined generally horizontal axis and in a predetermined directionto cause horizontal advancement of an airplane: said propeller having ablade projecting generally radially outwardly away from said axis andwhich has a leading edge, and a trailing edge, and a tip edge extendingtherebetween; said tip edge being disposed at a pitch angle with respectto a plane extending perpendicular to said axis of the propeller; saidleading and trailing edges having outer portions at substantially theiroutermost ends which extend approximately radially with respect to saidaxis and which essentially meet opposite ends of said tip edge; said tipedge being so shaped that, as it advances generally circularly from saidapproximately radial outer portion of the leading edge to saidapproximately radial outer portion of said trailing edge, the tip edgealso advances progressively radially outwardly away from said axis,throughout essentially the entire circular distance between saidapproximately radial outer portions of said leading and trailing edges,and at an angle of 71.5* with respect to a line extending directlyradially outwardly from said axis and intersecting the tip edge; saidblade in extending between said tip edge and said trailing edge havingan airfoil cross section in all planes perpendicular to a radial lineand intersecting both the tip edge and trailing edge, which airfoilcross section is defined by a convexly curved forward face of the bladeand a substantially flat rear face following a straight line betweensaid tip edge and said trailing edge; said airfoil cross section, in allof said planes, tapering to thin substantially sharp shapes at both thetip edge and trailing edge of the blade.
 2. A propeller which is to bepower driven rotatively about a predetermined generally horizontal axisand in a predetermined direction to cause horizontal advancement of anairplane: said propeller having a blade projecting generally radiallyoutwardly away from said axis and which has a leading edge, and atrailing edge, and a tip edge extending therebetween; said tip edgebeing disposed at a pitch angle with respect to a plane extendingperpendicular to said axis of the propeller; said leading and trailingedges having outer portions at substantially their outermost ends whichextend approximately radially with respect to said axis and whichessentIally meet opposite ends of said tip edge; said tip edge being soshaped that, as it advances generally circularly from said approximatelyradial outer portion of the leading edge to said approximately radialouter portion of said trailing edge, the tip edge also advancesprogressively radially outwardly away from said axis, throughoutessentially the entire circular distance between said approximatelyradial outer portions of said leading and trailing edges, and at anangle of 71.5* with respect to a line extending directly radiallyoutwardly from said axis and intersecting the tip edge; said blade inextending between said tip edge and said trailing edge having an airfoilcross section, in all planes perpendicular to a radial line andintersecting both the tip edge and trailing edge, which airfoil crosssection is defined by a convexly curved forward face of the blade and asubstantially flat rear face following a straight line between said tipedge and said trailing edge; said airfoil cross section, in all of saidplanes, being a fifty percent airfoil cross section having its maximumthickness at a point midway between said tip edge and said trailingedge, and tapering to thin substantially sharp shapes at both the tipedge and trailing edge of the blade.
 3. A propeller which is to be powerdriven rotatively about a predetermined generally horizontal axis and ina predetermined direction to cause horizontal advancement of anairplane: said propeller having a blade projecting generally radiallyoutwardly away from said axis and which has a leading edge, and atrailing edge, and a tip edge extending therebetween; said tip edgebeing disposed at a pitch angle with respect to a plane extendingperpendicular to said axis of the propeller; said leading and trailingedges having outer portions at substantially their outermost ends whichextend approximately radially with respect to said axis and whichessentially meet opposite ends of said tip edge; said tip edge being soshaped that, as it advances generally circularly from said approximatelyradial outer portion of the leading edge to said approximately radialouter portion of said trailing edge, the tip edge also advancesprogressively radially outwardly away from said axis, throughoutessentially the entire circular distance between said approximatelyradial outer portions of said leading and trailing edges, and at anangle of 71.5* with respect to a line extending directly radiallyoutwardly from said axis and intersecting the tip edge; said blade inextending between said tip edge and said trailing edge having an airfoilcross section, in all planes perpendicular to a radial line andintersecting both the tip edge and trailing edge, which airfoil crosssection is defined by a convexly curved forward face of the blade and asubstantially flat rear face following a straight line between said tipedge and said trailing edge; said airfoil cross section, in all of saidplanes, being a 50 percent airfoil cross section having its maximumthickness at a point midway between said tip edge and said trailingedge, and tapering to thin substantially sharp shapes at both the tipedge and trailing edge of the blade; said blade also having an airfoilcross section through at least a portion of its radial extent radiallyinwardly of said tip edge, in planes intersecting both said leading edgeand said trailing edge, which airfoil cross seciton is defined by aconvexly curved forward face and an essentially flat rear face followinga straight line between said leading edge and said trailing edge.